Semiconductor-based optical modulators have been developed using many different technologies and are used for modulating light for optical communications. One such semiconductor-based optical modulator is referred to as an electro-absorption modulator. In one example, a semiconductor electro-absorption optical modulator is used to modulate light in the 1550 nanometer (nm) wavelength for use in a long range fiber-optic communication system. A typical semiconductor electro-absorption optical modulator is fabricated using wafer processing technology. The optical modulator is typically reverse biased by an external electrical circuit so that when an input light source is coupled into the optical modulator, the input light is converted by one or more quantum wells in the optical modulator into a photocurrent. The wavelength of light at which the quantum wells in the optical modulator absorb photons and convert the photons to a photocurrent is dependent upon the material used to fabricate the layers of the optical modulator and the electrical bias applied to the optical modulator. The wavelength of light at which the quantum wells in the optical modulator absorb photons and convert the photons to a photocurrent can also be influenced by exploiting what is referred to as the Quantum Confined Stark Effect. The Quantum Confined Stark Effect is a phenomenon that allows the ground state subband energy separation of the material used to form the quantum well of the modulator to be reduced using a reverse electrical bias applied to the optical modulator. The reduction of the subband energy creates what is referred to as a “field effect” optical modulator. By employing the Quantum Confined Stark Effect, the speed at which an optical modulator can operate greatly exceeds the speed at which a conventional directly-modulated semiconductor laser can transmit data.
Generally, the bandgap of the material used to form the quantum well layer is lower than the bandgap of the material used to form the barrier layers that sandwich each quantum well layer. When the optical modulator is appropriately electrically biased, input light directed toward the quantum well is absorbed to generate electrical charge carriers, i.e., electrons in the conduction band and holes in the valence band, in the quantum well. The electron-hole pairs are then extracted from the quantum well to develop a photocurrent. The material used to form the quantum well layer and the electrical bias applied to the optical modulator greatly influences the absorption coefficient of the quantum well in the optical modulator. The absorption coefficient is a measure of the ability of the quantum well to absorb light and generate electron-hole pairs.
The material used to form the quantum well and the material used to form the barrier layers greatly influences the ability of the quantum well layer to release the photogenerated electron-hole pairs to generate the photocurrent. For example, a high energy barrier at the junction of the quantum well layer and the barrier layer provides a well-defined quantum state that exhibits a high absorption coefficient. However, a high energy barrier at the junction of the quantum well layer and the barrier layer also makes it difficult to extract the electron-hole pairs and generate a large photocurrent. If the photogenerated carriers are not efficiently extracted, an internal e-field will be created causing the response of the optical modulator to be slowed and causing the absorption characteristic to saturate with respect to incident power.
Therefore, it is desirable to provide an optical modulator that exhibits a high absorption coefficient and that generates a large photocurrent, while minimizing saturation and maintaining fast response at high optical power.